The large heat content of flue gases evolving from a high-temperature furnace such as that used in glass melting is primarily utilized in a metallic recuperator to preheat the combustion air of the furnace. In the glass-making industry it has become the practice to dispose such a recuperator in an elevated position above the level of the vitreous mass in order to allow entrained flux, such as borax, to drop back into the melt. The exit port of the recuperator, therefore, may lie as much as 10 to 20 meters above ground.
It has already been proposed to recover the still considerable residual heat of such flue gases with the aid of a waste-heat boiler comprising several vertical ducts traversed by the gases which leave the exit port of the recuperator. Mounting a boiler at such an elevated location requires a sturdy framework; its limited accessibility makes maintenance and inspection rather difficult and expensive.